1. Field of the Invention
This invention relates to apparatus for testing smoke detectors of both the ionization and the photoelectric type and more particularly to a novel such tester which involves actual simulation of fire conditions.
3. Brief Description of the Prior Art
There are two basic types of smoke detectors. One is the ionization detector, which senses changes in the conductivity of the air in a measuring chamber or chambers under the influence of radio-active radiation. The other is the photoelectric detector which senses the scattering of light in a measuring chamber. Both devices respond to the presence of particulate matter, or particles of combustion produced by thermal decomposition. The great majority of these particles, or aerosols, are invisible (less than 0.3 micrometers in size) but the larger ones are visible in the form of smoke (0.3 micrometers to 10.0 micrometers).
Invisible aerosol is the earliest appearing fire signature noted to date. Heating of materials during the pre-ignition stage of a fire produces submicron particles ranging in size from 5.times.10.sup.-4 to 1.times.10.sup.-3 micrometers. These particles are generated at temperatures well below ignition temperatures.
As heating of a material progresses toward the ignition temperature, the concentration of visible aerosol increases to the point where larger particles are formed by coagulation. As this process continues, the particle size distribution becomes log normal with the most frequent sizes in the range between 0.1 and 1.2 micrometers. This is the size range to which both ionization and photoelectric devices will respond, the photoelectric device reacting to sizes 0.3 micrometers and above and the ionization to the entire range and smaller. The smaller particles, less than 0.1 micrometers disappear either by coagulation or by evaporation, and the larger particles, greater than 1.0 micrometer, are lost through the processes of sedimentation following Stokes' Law. Aerosols in this size range are remarkably stable and contain particles in both the visible and invisible aerosol signature range. This "ageing" of aerosols has been reported by Van Luik and Scheidweiler.
Smoke detectors are a widely used home safety device which are designed to protect lives and to reduce property damage by giving a warning of fire when the fire is in its earliest stage in the home. If the home is equipped with one or more smoke detectors, the alarm, typically a loud horn, given by a properly functioning smoke detector can immediately alert the home occupants, giving them the time they need to safely exit the building. How fast the detector responds is of particular importance because the time interval between the warning alarm and the spread of the fire through the household is the critical factor. A few minutes is often the difference between survival and death in the typical home fire.
Detectors may fail to alarm as required because their sensitivities have been altered over a period of time by dust, grease, corrosive fumes, moisture or by other contaminants in the area in which the detector is located. Electronic component failures are also known to occur. Aging, as well, is a factor contributing to malfunctions for these reasons. Detector manufacturers typically provide for "testing" the device by means of pressing a test button or by pulling a switch which is located on the housing of the unit. Alternatively, some (older) models can only be tested by blowing smoke in the direction of the detector, i.e., through smoke derived from a cigarette, cigar, match, candle, paper, rope, etc.
There are major disadvantages in the conventional testing of smoke detectors. Recommended placement of the smoke detector is on the ceiling or high-up on the wall. A person of average height must stand on a chair or on some elevation in order to press the button, pull the switch, or blow the smoke, thus risking physical harm, which is a serious matter for older people. At best, blowing smoke is a clumsy and primitive method for testing such devices.
The only true test for a smoke detector is one that involves creating the particular matter (aerosols) which simulates the advance or early products of combustion. Underwriters Laboratories, Inc. publishes a Standard for Safety for Single and Multiple Station Smoke Detectors (UL 217) which specifies the detection levels for an approved smoke detector. Compliance is determined by empirical tests involving, among others, flaming paper, gasoline and smoldering wood. A detector must function satisfactorily in all such tests. The levels of performance required are minimum standards, basically calling for any detector to operate an alarm when exposed to particle sizes in the range of 0.1 micrometers to 1.2 micrometers. See FIG. 4. So it is important that in testing smoke detectors the test should provide only those detectable elements within the prescribed minimum levels for an approved detector. It would be improper and ineffective to employ an "overkill" type test, such as used by Gustav Purt, et al (U.S. Pat. No. 3,693,401) wherein a housing encloses the smoke detector, creating an artificial environment into which aerosols of indeterminate size are introduced in a gross quantity. Purt's artificial environment, of course, precludes normal air flow conditions and the aerosols introduced are not limited as to quantity or appropriate size.